Conventionally, a high intensity discharge lamp (HID lamp) such as a metal halide lamp is used in a vehicle headlamp because of its high luminous flux. The discharge lamp used in a vehicle headlamp is turned on by a discharge lamp lighting device that converts a DC power inputted from a battery into an AC power.
FIG. 10 shows a circuit configuration of a conventional discharge lamp lighting device. In the discharge lamp lighting device, a DC-DC converter 104 converts a DC voltage of a DC power source into a predetermined DC power. The DC power is converted by an inverter 105 into a low-frequency alternating square wave power and then is supplied to the discharge lamp La via a starting circuit 130. The DC-DC converter 104 is formed of a flyback converter, and the DC power supplied to the inverter 105 is controlled by adjusting a pulse width modulation (PWM) signal to drive a switching element Q0 connected in series with a primary winding of a transformer T.
The inverter 105 has full-bridged switching elements Q1 to Q4. The inverter 105 converts a DC power from the DC-DC converter 104 into an alternating square wave power by alternately turning on and off a pair of switching elements Q1 and Q4, and a pair of switching elements Q2 and Q3. At the startup of the discharge lamp La, the starting circuit 130 supplies a pulse current to a primary side of a pulse transformer PT by a pulse drive circuit 131, and applies a high voltage generated on a secondary side of the pulse transformer PT according to a ratio of the number of coil windings, to the discharge lamp La, thereby initiating the discharge lamp La.
In the discharge lamp lighting device configured as such, a low-frequency alternating square wave power is supplied to the discharge lamp La from the inverter 105 in order to avoid acoustic resonance phenomena. In the polarity inversion of the alternating square wave power, an output currents I1a (lamp current) passes through a zero point. Accordingly, the discharge stops at a moment when the polarity of the output current I1a is inverted. Then, in order for the output current I1a to pass the zero point and begin flowing in the reverse direction, it is necessary to apply a predetermined high voltage generally referred to as “re-ignition voltage” to the discharge lamp La.
As shown in FIG. 11, when an output voltage Vo of the inverter 105 is inverted, the output current I1a is also inverted. Since there is a secondary side inductance Lp in the pulse transformer PT of the starting circuit 130, it is impossible to sharply change the lamp current I1a like the voltage Vo. That is, the output current I1a is inverted at a predetermined slope dI1a/dt.
It is necessary to increase the re-ignition voltage as the slope dI1a/dt of the lamp current I1a in the polarity inversion gets smaller. If the re-ignition voltage is not sufficiently supplied from the inverter 105, as shown in FIG. 12, a time period Ts is generated in which the lamp current I1a is zero or lower than normal. As a result, a noise may occur in the pulse current, or the lifespan of the discharge lamp La may be reduced. Further, when the time period Ts becomes longer, it may cause flickering or turn off.
Accordingly, in the discharge lamp lighting device, by increasing the output of the DC-DC converter 104 and increasing the output voltage Vo of the inverter 105 in the polarity inversion, a required re-ignition voltage is ensured. Specifically, the discharge lamp lighting device 1 includes a PWM ON signal controller 109 to increase the DC power. The PWM ON signal controller 109 extends the ON duty of the PWM signal during a predetermined period from the start of the polarity inversion, in the switching conditions immediately before the polarity of the alternating power is inverted.
The PWM ON signal controller 109 includes an edge detection/one-shot pulse circuit 191 and an ON duty increase circuit 192. The edge detection/one-shot pulse circuit 191 detects a rising edge and a falling edge of a signal outputted from a low frequency oscillation circuit (LF-OSC) of an inverter driving signal generator 106 and generates a pulse signal with a predetermined width. The ON duty increase circuit 192 outputs a signal to increase the ON period of the switching element Q0 in the duration of the pulse width of the PWM signal outputted from the PWM signal generator 108, so that the output of the DC-DC converter 104 is increased. By this configuration, the discharge lamp lighting device can ensure the sufficient re-ignition voltage by increasing the output voltage of the inverter 105 (see, e.g., Japanese Patent Laid-open Publication No. 2010-231995).
In the discharge lamp lighting device described in Japanese Patent Laid-open Publication No. 2010-231995, a predetermined amount of the output voltage is increased regardless of an initial use state or an end-of-life state. Accordingly, in the initial state of the discharge lamp in which the lamp voltage is low, there is a possibility of increasing the stress which is applied to the discharge lamp, worsening the switching noise, or shortening the lifespan of the discharge lamp or the like.